The plasma membrane of living cells is an important structure that regulates the movement of solutes and maintains a cellular solute composition very different from that of the external environment. Ions, nutriments and water are transported through integral membrane proteins classified into several families, the members of which are found in all living species. One of these transport protein families is the major intrinsic protein family (MIP), named after the first member described, which was found in mammalian lens tissue. MIP proteins have a mass of approximately 30 kDa and are characterized by 6 transmembrane-spanning helices, cytosolic amino and carboxy termini and the signature sequence SGxHxNPAVxT which is repeated in the second half of the protein as NPA (Park et al., J Membr Biol 153:171-180 (1996); Maurel et al., Proc. Natl. Acad. Sci. USA 94: 7103-7108 (1997): Agre et al., J. Biol. Chem. 273:24 14659-14662, (1998). Many MIP proteins are aquaporins, which enhance the membrane permeability to water, but some form channels for small neutral solutes, including glycerol and urea.
Aquaporins are involved in: cell expansion and cell volume regulation, transcellular water flow, adjustments to water deficit, regulated water delivery, and adaptation to cold stress by osmotic adjustments. Some aquaporins have been shown to be dessication and salt stress induced in plants. Yamaguchi-Shinozaki et al., Plant Cell Physiol., 33: 217-224, 1992; Guerrero et al., Plant Mol. Biol., 15: 11-26, 1990. Kaldenhoff et al., (Plant J., 7: 87-95, (1995)) showed that the down regulation of aquaporins in Arabidopsis plants that express antisense constructs results in a slower swelling and bursting of the isolated protoplasts.
Tonoplast Intrinsic Proteins (TIP) are MIP homologs that have been identified in seeds from a wide range of monocot and dicot species. Johnson et al., Plant Physiology, 91: 1006-13, 1989. Full or partial sequences have been determined in bean (Johnson et al., Plant Cell, 2: 525-32, 1990), Arabidopsis thaliana (Hofte et al., Plant Physiology, 99: 561-70, 1992), pumpkin (Cucurbita sp.) (Inoue et al., Plant Molecular Biology, 28: 1089-1101, 1995), and Norway spruce (Picea abies) (Oliviusson and Hakman, Physiologia Plantarum, 95: 288-95, 1995) and confirm the high conservation of these proteins in seeds. Full or partial sequences of more than 20 MIP homologs in Arabidopsis are reported to be available in the sequence data banks. Maurel et al., J. Exp. Botany, 48: Special Issue, 421-430, 1997.
An important paramater of crop plants is water use efficiency. Plants able to make better use of scarce water resources promise to make agriculture more sustainable in many areas of the U.S. and the world. Accordingly, what is needed in the art are plants which are dessication-, salt-, cold-, or drought-tolerant. The present invention provides these and other advantages.